Arrangement of Electrons in Atoms
The Development of a New Atomic Model
The Development of the Atom
Dalton’s model
Characteristics
Dalton’s atomic theory
Modifications to Dalton’s atomic theory
Thomson’s model
Characteristics
Grapes in jello
Plum pudding model
Rutherford’s model
Gold foil experiment
Characteristics
Problems
What was the exact location of electrons?
Why did electrons not fall into the nucleus?
Properties of Light
The study of light revealed the relationship between it and an atom’s electrons.
Light is a form of electromagnetic radiation(EMR).
It is a form of energy.
Other forms of EMR are gamma rays, X-rays, microwaves, ultraviolet rays, infrared rays, and radio waves.
Speed is 3 x 108 m/s(c) in a vacuum.
It has wave-like features.
The Electromagnetic Spectrum
Wave Features
Repetitive nature.
Wavelength – distance from crest to crest(m, cm, nm)()
Frequency – how many crests pass a point in one second(cps, Hz)(f or )
1cps = 1Hertz
Crest - peak
Trough – valley
Wavelength and Frequency
The Photoelectric Effect
The photoelectric effect refers to the emission of electrons from a metal when light shines on the metal.
The effect depends on the frequency of the light striking the metal. If the frequency is too low, nothing happens regardless of time of exposure or intensity. A certain minimum frequency was required.
This was a problem because classical wave theory physics predicted that any frequency of light would cause the effect.
The Particle Description of Light
Planck was studying the emission of light by hot objects(black body radiators).
He proposed that such objects emitted energy in small, specific packets of energy(quanta).
His relationship was E = h or hf, where E is the energy in joules, h is Planck’s constant(6.626x 10-34 Js) and or f is frequency.
Einstein expanded on Planck’s ideas and introduced the idea that EMR had a dual wave-particle nature.
According to him, light could travel in both wave motion and particle motion.
He called particles of light photons. Each photon carried a quantum of energy.
Einstein explains the photoelectric effect
Einstein proposed that EMR is absorbed by matter only in whole numbers of photons. In order for an electron to be ejected from a metal surface,the electron must be struck by a single photon possessing the minimum energy required to knock the electron loose. This energy corresponds to a threshhold frequency. If the frequency is too low, nothing happens. If the frequency is too high, more energetic electrons are emitted. If the intensity of the light is increased at the threshhold frequency, more electrons are emitted.
The Hydrogen Atom Line Emission Spectrum
When current is passed through a gas at low pressure, the potential energy of some gas atoms increases.
The lowest energy state of an electron is the ground state.
A state in which an atom has a higher potential energy than it has in its ground state is an excited state.
When an electron returns to the ground state, it emits absorbed energy in the form of light. When this light was passed through a prism, a bright line emission spectrum was received.
Hydrogen’s Line Emission Spectrum(Balmer series lines)
Hydrogen Atom Line Emission Spectrum
Classical physics predicted that the hydrogen atoms would be excited by any amount of energy and that their electrons could be in any energy state.
If that was true, then a continuous spectrum(rainbow) would be seen.
As shown, only specific frequencies of light are given off.
This was even the case in non-visible spectra from the infrared and ultraviolet regions of the EMR spectrum.
Bohr’s model of the
hydrogen atom
In 1913, Bohr proposed a model of the hydrogen atom linking the atom’s electron with photon emission.
The electron can circle the nucleus in allowed orbits only. It has fixed energy.
It has lowest energy closest to the nucleus. As it moves away from the nucleus, it gets higher in energy.
There is a common analogy between Bohr’s atom and the rungs of a ladder.
What really happens when energy
is absorbed and re-emitted?
The Quantum Model of the Atom
New developments – De Broglie
De Broglie wavelength
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